Recently, with the development of electrical technology, low voltage and high current applications are widely used. Low voltage operation helps to reduce power loss, but also raises new challenge to power supply.
There are three main components, i.e., power switch, transformer and rectifying diode, contributed to power loss for a switching mode power supply. A voltage drop on the rectifying diode is relatively high at low voltage applications. As a result, power loss introduced by the rectifying diode is relatively large. For example, voltage drop on a fast recovery diode (FRD) or an ultra-fast recovery diode (SRD) may be about 1.0V-1.2V, and voltage drop on a schottky diode may be about 0.6V.
Synchronous rectification is a technology for reducing power loss on rectifying device and improving efficiency by replacing rectifying diode with power metal oxide semiconductor field-effect transistor (MOSFET). Generally speaking, on-resistance Rds(on) of MOSFET is relatively low to improve efficiency of switching mode power supply at low voltage applications and there is no dead zone introduced by schottky barrier voltage. MOSFET is a voltage controlled device and MOSFET has a linear voltage-current characteristic when turned ON. Gate voltage of a rectifying MOSFET needs to be in phase with a rectified voltage for synchronous rectification.
Traditional control methods for synchronous rectification adopt discrete self-driven, single-chip phase-locked loop and smart rectifier. Disadvantages of discrete self-driven method for synchronous rectification are slow response and low system reliability. Single-chip phase-locked loop for synchronous rectification is configured to control on/off of the rectifying MOSFET based on signal at primary side. Disadvantage of single-chip phase-locked loop method for synchronous rectification is low reliability in burst mode, i.e., when light load or no load occurs. The best method is smart rectifier method, which is independent on signal at primary side. Smart rectifier method detects voltage drop on the rectifying MOSFET directly and has quick response.
FIG. 1 shows waveforms illustrating signals of traditional smart rectifier. Take a switching mode power supply comprising a transformer as an example. As shown in FIG. 1, a drain-source voltage Vds of a rectifying switch, a current signal Isec indicating current flowing from the secondary winding to a load, and a drive signal DRV of the rectifying switch are illustrated. Drain-source voltage Vds is employed to compare with a threshold signal Vth1 and a threshold signal Vth2. When a body diode of the rectifying switch is turned ON, drain-source voltage Vds decreases rapidly. If drain-source voltage Vds decreases less than threshold signal Vth2, the rectifying switch will be turned ON. If drain-source voltage Vds rises larger than threshold signal Vth1, the rectifying switch will be turned OFF.
A disadvantage of traditional smart rectifier is that shoot-through may occur under some conditions. For example, per characteristics of the rectifying switch and/or delay of a control circuit, after drain-source voltage Vds rises up to threshold Vth1, there may be a turn OFF delay time period to turn OFF the rectifying switch and there may be a residual current transferring from a secondary winding to a primary winding. If the turn OFF delay time period is long, the rectifying switch may be not turned OFF in time, and the rectifying switch and a switch at primary side may be turned ON at the same time. As a result, shoot-through occurs and the switching mode power supply is under the danger of broken down.
Thus, an improved synchronous rectifying control method is needed.